hypercalls.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3===================
  4Linux KVM Hypercall
  5===================
  6
  7X86:
  8 KVM Hypercalls have a three-byte sequence of either the vmcall or the vmmcall
  9 instruction. The hypervisor can replace it with instructions that are
 10 guaranteed to be supported.
 11
 12 Up to four arguments may be passed in rbx, rcx, rdx, and rsi respectively.
 13 The hypercall number should be placed in rax and the return value will be
 14 placed in rax.  No other registers will be clobbered unless explicitly stated
 15 by the particular hypercall.
 16
 17S390:
 18  R2-R7 are used for parameters 1-6. In addition, R1 is used for hypercall
 19  number. The return value is written to R2.
 20
 21  S390 uses diagnose instruction as hypercall (0x500) along with hypercall
 22  number in R1.
 23
 24  For further information on the S390 diagnose call as supported by KVM,
 25  refer to Documentation/virt/kvm/s390/s390-diag.rst.
 26
 27PowerPC:
 28  It uses R3-R10 and hypercall number in R11. R4-R11 are used as output registers.
 29  Return value is placed in R3.
 30
 31  KVM hypercalls uses 4 byte opcode, that are patched with 'hypercall-instructions'
 32  property inside the device tree's /hypervisor node.
 33  For more information refer to Documentation/virt/kvm/ppc-pv.rst
 34
 35MIPS:
 36  KVM hypercalls use the HYPCALL instruction with code 0 and the hypercall
 37  number in $2 (v0). Up to four arguments may be placed in $4-$7 (a0-a3) and
 38  the return value is placed in $2 (v0).
 39
 40KVM Hypercalls Documentation
 41============================
 42
 43The template for each hypercall is:
 441. Hypercall name.
 452. Architecture(s)
 463. Status (deprecated, obsolete, active)
 474. Purpose
 48
 491. KVM_HC_VAPIC_POLL_IRQ
 50------------------------
 51
 52:Architecture: x86
 53:Status: active
 54:Purpose: Trigger guest exit so that the host can check for pending
 55          interrupts on reentry.
 56
 572. KVM_HC_MMU_OP
 58----------------
 59
 60:Architecture: x86
 61:Status: deprecated.
 62:Purpose: Support MMU operations such as writing to PTE,
 63          flushing TLB, release PT.
 64
 653. KVM_HC_FEATURES
 66------------------
 67
 68:Architecture: PPC
 69:Status: active
 70:Purpose: Expose hypercall availability to the guest. On x86 platforms, cpuid
 71          used to enumerate which hypercalls are available. On PPC, either
 72	  device tree based lookup ( which is also what EPAPR dictates)
 73	  OR KVM specific enumeration mechanism (which is this hypercall)
 74	  can be used.
 75
 764. KVM_HC_PPC_MAP_MAGIC_PAGE
 77----------------------------
 78
 79:Architecture: PPC
 80:Status: active
 81:Purpose: To enable communication between the hypervisor and guest there is a
 82	  shared page that contains parts of supervisor visible register state.
 83	  The guest can map this shared page to access its supervisor register
 84	  through memory using this hypercall.
 85
 865. KVM_HC_KICK_CPU
 87------------------
 88
 89:Architecture: x86
 90:Status: active
 91:Purpose: Hypercall used to wakeup a vcpu from HLT state
 92:Usage example:
 93  A vcpu of a paravirtualized guest that is busywaiting in guest
 94  kernel mode for an event to occur (ex: a spinlock to become available) can
 95  execute HLT instruction once it has busy-waited for more than a threshold
 96  time-interval. Execution of HLT instruction would cause the hypervisor to put
 97  the vcpu to sleep until occurrence of an appropriate event. Another vcpu of the
 98  same guest can wakeup the sleeping vcpu by issuing KVM_HC_KICK_CPU hypercall,
 99  specifying APIC ID (a1) of the vcpu to be woken up. An additional argument (a0)
100  is used in the hypercall for future use.
101
102
1036. KVM_HC_CLOCK_PAIRING
104-----------------------
105:Architecture: x86
106:Status: active
107:Purpose: Hypercall used to synchronize host and guest clocks.
108
109Usage:
110
111a0: guest physical address where host copies
112"struct kvm_clock_offset" structure.
113
114a1: clock_type, ATM only KVM_CLOCK_PAIRING_WALLCLOCK (0)
115is supported (corresponding to the host's CLOCK_REALTIME clock).
116
117       ::
118
119		struct kvm_clock_pairing {
120			__s64 sec;
121			__s64 nsec;
122			__u64 tsc;
123			__u32 flags;
124			__u32 pad[9];
125		};
126
127       Where:
128               * sec: seconds from clock_type clock.
129               * nsec: nanoseconds from clock_type clock.
130               * tsc: guest TSC value used to calculate sec/nsec pair
131               * flags: flags, unused (0) at the moment.
132
133The hypercall lets a guest compute a precise timestamp across
134host and guest.  The guest can use the returned TSC value to
135compute the CLOCK_REALTIME for its clock, at the same instant.
136
137Returns KVM_EOPNOTSUPP if the host does not use TSC clocksource,
138or if clock type is different than KVM_CLOCK_PAIRING_WALLCLOCK.
139
1407. KVM_HC_SEND_IPI
141------------------
142
143:Architecture: x86
144:Status: active
145:Purpose: Send IPIs to multiple vCPUs.
146
147- a0: lower part of the bitmap of destination APIC IDs
148- a1: higher part of the bitmap of destination APIC IDs
149- a2: the lowest APIC ID in bitmap
150- a3: APIC ICR
151
152The hypercall lets a guest send multicast IPIs, with at most 128
153128 destinations per hypercall in 64-bit mode and 64 vCPUs per
154hypercall in 32-bit mode.  The destinations are represented by a
155bitmap contained in the first two arguments (a0 and a1). Bit 0 of
156a0 corresponds to the APIC ID in the third argument (a2), bit 1
157corresponds to the APIC ID a2+1, and so on.
158
159Returns the number of CPUs to which the IPIs were delivered successfully.
160
1618. KVM_HC_SCHED_YIELD
162---------------------
163
164:Architecture: x86
165:Status: active
166:Purpose: Hypercall used to yield if the IPI target vCPU is preempted
167
168a0: destination APIC ID
169
170:Usage example: When sending a call-function IPI-many to vCPUs, yield if
171	        any of the IPI target vCPUs was preempted.
172
1739. KVM_HC_MAP_GPA_RANGE
174-------------------------
175:Architecture: x86
176:Status: active
177:Purpose: Request KVM to map a GPA range with the specified attributes.
178
179a0: the guest physical address of the start page
180a1: the number of (4kb) pages (must be contiguous in GPA space)
181a2: attributes
182
183    Where 'attributes' :
184        * bits  3:0 - preferred page size encoding 0 = 4kb, 1 = 2mb, 2 = 1gb, etc...
185        * bit     4 - plaintext = 0, encrypted = 1
186        * bits 63:5 - reserved (must be zero)
187
188**Implementation note**: this hypercall is implemented in userspace via
189the KVM_CAP_EXIT_HYPERCALL capability. Userspace must enable that capability
190before advertising KVM_FEATURE_HC_MAP_GPA_RANGE in the guest CPUID.  In
191addition, if the guest supports KVM_FEATURE_MIGRATION_CONTROL, userspace
192must also set up an MSR filter to process writes to MSR_KVM_MIGRATION_CONTROL.